Electric constant temperature device



Sepf. 21,1937. EJP. L. WESTELL ELECTRIC CONSTANT TEMPERATURE DEVICEFiled llay 26, 1936 3 Sheets-Sheet 1 Sept. 21, 1937.

E. P. WESTELL Y ELECTRIC CONSTANT TEMPERATURE DEVICE Filed May 26, 19363 Sheets-Sheet 2 //V VEN TOR ATTORNEY Sept. 21, 1937. E. P. L. WESTELLELECTRIC CONSTANT TEMPERATURE DEVICE Filed May 26 1936 3 Sheets-Sheet 3Fig. 3.

lr-HT Patented Sept. 21, 1937 Application May 26,

ELECTRIC CONSTANT TEMPERATURE DEVICE Edgar Philip Lawrence Westell,Coventry, England, assignor to The General Electric Company Limited,London, England 1936, Serial, No. 81,969

In Great Britain May 31, 1935 5 Claims.

This invention relates to constant temperature device of the type inwhich the thermally sensitive element is a Wheatstone bridge whose armsare constituted so that the bridge balances at one temperature only.

In such constant temperature device a supply is usually connected acrossone diagonal and a responsive element (for instance, a relay) across theother. The responsive element controls a current heating the constanttemperature device. It has been proposed to include a thermionicamplifier in the responsive element, and, in particular, to connect thegrid and cathode of a thermionic triode directly across the diagonal andto use the anode current to control, directly or through a relay, theheating of the constant temperature device. The object of the inventionis to provide an improved modification ofsuch known arrangements, whichavoids all moving parts within the controlled space and in which thesensitivity of control is substantially greater than hitherto achieved.v The principle on which the invention depends is that, if the input ofan amplifier is connected across one diagonal of a bridge and the outputoi the amplifier across another, there will be no coupling between inputand output through the bridge so long as the bridge is balanced; butwhen the bridge is unbalanced, there will be coupling through thebridge. In order that the amplifier may be capable of self-maintainedoscillations, it

is necessary that there should be more than some limiting amount ofcoupling between its input and output. If it is arranged that this limitis attained when the bridge is unbalanced, but not when it is balanced,unbalance of the bridge due to departure of the constant temperaturedevice from the desired temperature will set the amplifier intooscillation. The presence of these oscillations can be made to supply orsubtract heat from the constant temperature device so as to restore itto the desired temperature.

One simple application of this principle is illustrated in Figure 1 ofthe accompanying drawings;

Figure 2 shows a more complete diagram and Figure 3 anothermodification;

In Figure I the bridge has two opposite arms R of one material and twoother opposite arms R1; of another material, these materials havingdifferent temperature coefiicients of resistance.

The bridge is constituted so that it is balanced at some desiredtemperature To of the constant temperature device. At this temperaturethere will be no coupling through the bridge and no oscillation of theamplifier. When the constant temperature device is at a temperature Tsufilciently different from To, the bridge is unbalanced, and a voltage11. across the output of the amplifier 2 produces a voltage v across itsinput. If v /v. is sufiiciently large and or the right sign (accordingto known principles), this coupling between input and output will causethe amplifier 2 to oscillate and the relay 1 to operate.

It is to be observed, however, that if v /vt is of the right sign when Tis greater than To, it will generally be of the wrong sign when T isless than To and vice versa. Accordingly the amplifier will oscillateand the relay will operate only for changes in temperature in onedirection. 'Even if this limitation could be removed and the relay madeto operate for changes in temperature in either direction, there wouldbe nothing to dis tinguish operation caused by oscillations generated byT becoming greater than To from operation caused by T becoming less thanTo. Accordingly it is impossible to arrange that when T becomes greaterthan To, the relay cuts off heat so as to oppose the change and also toarrange that when T becomes less than To the relay supplies heat tooppose the change. The arrangement can compensate change in onedirection only.

This feature appears to be inherent in the use of the principlecharacteristic of the invention. It must, therefore, always beunderstood that the constant temperature device is one in which T tendsto depart from To in one direction only, that is to say, to rise aboveTo but not to fall, or to fall but not to rise.

In Figure 1 the relay 1 starts a current in the circuit 8 fed by thesupply I and containing the heating element 9 by which heat is suppliedto the constant temperature device. The constant temperature devicemust, therefore, be one in which T tends to fall below To and the bridgemust be arranged so that, when T falls below To, v /v. is of the rightsign to enable the amplifier to oscillate. If the constant temperaturedevice is one in which T tends to rise above To, two changes must bemade. First the sign of 'Ug/Da for a given temperature change must bereversed; this can be achieved by reversing the connections between (3,3') and (4, 4) or those between (5, 5) and (6, 6). Second, the relay 1,when it operates, must cut ofl the heating current from the element 9.

The control of the heating by the oscillation of theamplifler isindirect in Figure 1; for the heating and amplifier circuits areconnected only through the relay 1. But if the amplifier is able toiurnish sumcient power, the relay may be abolished and the control madedirect, that is to say, the heating current may be derived from theamplifier circuit. In this case the heating element through which theheating current flows need not be a separate resistance, as shown at 9;it may be constituted by some or all of the arms of the bridge. Constanttemperature devices are already known in which the same resistancesserve both as a thermally sensitive bridge and as a heating element.Direct control has the advan:

tage that it may be made continuous and not intermittent.

We have now to consider more closely how the oscillations are to controlthe heating. Here it 16 may be assumed that, in the present state of theart, the amplifiers will always include thermionic valves.

The simplest method in principle would be to use the alternatingcomponent of the amplifier outputs to operate the relay, if indirectcontrol were used, or to feed the heating element, if direct controlwere used. (Direct control could then be used only in constanttemperature devices in which T tends to fall below To.) But it isgenerally more convenient to use direct current for control. This directcurrent may be the mean anode current of one or more of the valves. Itmight be made to change when the valve starts to oscillate by causingthe representative point oi the valve in the non-oscillating state tolie on a curved part of its characteristic But preferably theoscillations are made to feed a rectifier by which grid bias isgenerated; this grid bias applied to one or more of the valves changesits mean anode current. The advantage of this method is that the sign ofthe grid bias generated and therefore the sign of the change in meananode current produced by the oscillations are easily adjusted to theneeds of the particular case.

When compensation can be applied only when T departs from T0 in onedirection, it is known to be desirable that compensation, once startedby variation of T, should continue even after (TTo) has been made lessthan the value which originally started the compensation. Since thiscondition always obtains in constant temperature devices according tothe invention, it is desirable that appropriate means for achieving suchcontinuation should be provided. For this reason also control by meansof grid bias generated as aforesaid is desirable; for by applying asuitablegrid bias the degree of coupling (and therefore the value ofTTo) necessary in order that 55 the amplifier may oscillate can bereduced. But other means obvious to experts are possible. Thus, ifindirect control is used, the relay may be biased towards the operatedstate by oscillations once started; or, conversely, the operation of therelay may apply a potential (e. g. a grid bias) to the amplifier biasingit towards oscillation. X

Preferably the amplifier is tuned to a single frequency, the arms of thebridge are non-reactive, and the amplifier is such that duringoscillation the phase difference between input and output is 0 or 180.But this is not essential; all that is necessary is that at the desiredtemperature the bridge shall be balanced at the same time for allfrequencies with which the amplifier can oscillate, and shall beunbalanced for some at least of these frequencies for all temperatureson some range on that side of the desired temperature towards which theconstant temperature device is apt to depart.

Two embodiments of the invention, which include some features notmentioned in the foregoing account of general principles, will now bedescribed with reference to Figures 2 and 3 of the accompanyingdrawings. Both figures are conventional circuit diagrams; Figure 2 showsindirect control, Figure 3 direct control.

In Figure 2, the bridge I, constructed as in Figure 1, is enclosed inthe constant temperature chamber The points 4, 4' areconnected (as inFigure 1) to 3, 3', which are the terminals oi the primary of an inputtransformer l2, whose secondary is connected between grid I3 and thecathode M of the thermionic triode IS; the connection to the cathodeincludes the source of constant grid bias I6 and the part II of apotentiometer resistance |8 which (as explained below) provides avariable grid bias IS. The source of constant bias is a resistance inthe heating circuit of cathodes l4 and 26 (see below) fed from the L. T.(low tensioni'supply 50, 5|, of which the negative terminal is joined tothe-H. T.

I (high tension) negative. A regulating resistance It is included inthis heating circuit. A smoothing condenser is connected across the partI! of the potentiometer H3. The anode l9 of the "triode I5 is connectedthrough the choke 20 and one winding of the relay 2| to the positiveterminal of the supply whose negative terminal is connected to earth orgrounded. The anode is also connected through the coupling condenser 22to a tapping" on the choke 23, which forms the tuning element inconjunction with the condenser 43 shunted across it. One end of thischoke is connected to the grid 24 of another thermionic triode 25, theother end at a point on the circuit by which the cathode 26 ofthistriode, as well as I4, is heated. The anode 29 of the triode 2 5 isconnected through the primary 42 of a transformer 52 and through theother winding of relay 2| to the positive terminal. Condensers 3| areprovided as usual to bye-pass A. C. components of the anode current, andprevent them passing through the windings of the relay 2|. 400 P. P. S.(periods per second) is a suitable natural frequency for the tunedelement.

The transformer 52 has two secondaries 33, 34. The secondary 33 is splitinto two halves, the inner ends of which, shunted by a condenser 53, areconnected to the plus and minus sides of a high tension circuit H; T+and H. T;.the outer ends, which correspond to 5, 5' in Figure 1, areconnected to the diagonal points 6, 6' of the bridge. The bridge is thusfed with D. C. current (for a purpose that will appear presently) and,when the amplifier oscillates, with A. C. current. Secondary 34 feeds arectifier 35 which in turn supplies current to the potentiometer Ill.

The relay 2| has contacts 38 which, on closing,

, enable current to flow through the heating element 36, enclosed in thechamber H, from the supply 31.

The primary winding 3, 3' is split and the inner ends, shunted by acondenser 54, are connected through a D. C. instrument 56, which thusreceives the D. C. out of balance current from the bridge I. Theinstrument 56 may be a measuring instrument, which will then act as adistant reading thermometer. Alternately or additionally it may be arelay which operates an alarm and breaks part of the circuit, if theapparatus ceases to function and the temperature of the bridge departswidely from that desired. But such devices do not form part of theinvention.

' flowing through the bridge.

The windings'oi' relay 2i oppose each other.

' and are adjusted so that, when the amplifier is not oscillating, theanodev currents of triodes ll ondary 34 produce a grid bias in the partll of the potentiometer making the grid II more 7 positive. The anodecurrent of the trlode II increases, the balance of the windings of relay2i is upset, relay 2| operates and contacts 38 close. Current istherefore supplied to the heating element 36 which causes thetemperature to rise till the oscillations cease.

v In this arrangement the positive grid bias applied to the grid l3makes the amplifier more ready to .osciliate and therefore fulfills thepurpose, above mentioned, of continuing oscillations after theoriginating disturbance has been renewed. In order to increase thiseffect, the valve It may be a varlable-mu valve.

Figure 3 shows an arrangement for direct control in which there is norelay, and compensation is effected by the anode current of a valveParts represented by the same reference numeral in Figures 2 and 3perform the same function. Accordingly only the differences need bementioned.

Relay 2| and the circuit and heating coils 36, 31, 38 are absent. Theeffective output terminals 5, 5' are now connected with the anode andcathode of the triode 25. This anode circult, which includes the bridgebetween 6 and 6', is fed by the supply 31, of which the negativeterminal is grounded; the supply 31 may be the same as that supplyingthe valve ii. The primary 42 of the transformer which feeds thesecondary 34 and rectifier 35 is connected in series with a condenser 4|across the output terminals. A condenser 46 and a choke 41 preventalternating current from reaching the supply. A blocking condenser 48 isinserted in the leads between the input and the bridge.

The heating circuits for cathodes l4 and 26 are not shown. Grid 24 hasnow a source of constant grid bias 21 which, like it, is shown as almttery; both grid 24 and grid l3 receive a variable bias from thepotentiometer It; the grid i3 is fed from the part l1 as before, thegrid 24 from the part 23. Accordingly when the oscillations start, themean anode current through both valves i5 and 25 increases; the currentthrough valve 25 alone flows through the bridge; the object ofincreasing the positive bias on valve it has been explained already.

It will usually be desirable to supply the bridge, which is now theheating element, with a constantcurrent, independent of thecompensation. This can be drawn from they supply 31 through the choke 44and variable resistance 45, one end of which is grounded.

Thermostats according to the invention are particularly suitable foraccurate control of temperatures but little above atmospheric. Thus theycan be used to control a temperature of C. to within i 0.05 C. if thesupply voltage is reasonably constant.

a; aoeans relay; for amplifiers can be designed whose gain is almostindependent of the supply voltage. But fullindependence of the supply 31feeding the heating element is not secured either in Figure 2 or Figure3.

I claim:-

1. A constant temperature device comprising a thermally sensitiveWheatstone bridge arranged to balance at a selected temperature and tobe come unbalanced by a change from said temperature, a trlode tubeamplifier having its input and output terminals connected to the ends ofopposite diagonals of said bridge so that a coupling is established whenthe bridge is unbalanced, circuits and connections adapted to start andmaintain oscillations in the amplifier when the bridge becomesunbalanced by a change from said selected temperature, heating means forcontrolling the temperature of the bridge and means controlled byoscillations of the amplifier for operating said heating means torestore the balance of the bridge and stop said oscillations.

2. A constant temperature device comprising a thermally sensitiveWheatstone bridge arranged to balance at a selected temperature and tobecome unbalanced by a change from said temperature, a trlode tubeamplifier having its input and output terminals connected to the ends ofopposite diagonals of said bridge so that a coupling is established whenthe bridge is unbalanced, circuits and connections adapted to start andmaintain oscillations in the amplifier when the bridge becomesunbalanced by a change from said selected temperature, an electricheating coil for restoring the temperature of the bridge when it dropsbelow the selected temperature and a relay adapted to be operated byoscillations of said amplifier for throwing in said heating coil, therelay I is unbalanced, means causing the amplifier to oscillate when thebridge becomes unbalanced by a change in temperature, a rectifieradapted to be operated by the alternating component in the output ofsaid amplifier to provide grid bias for at least one of said tubes forcontrolling the oscillations and means rendered effective by said bridgeand stopping the oscillations.

4. A constant temperature device as in claim 3, wherein the means forrestoring the temperature of the bridge comprises an electric heatingcoil adjacent the bridge which coil is adapted to be operated by a relaycontrolled by said oscillations in the amplifier.

5. A constant temperature device as in claim 3, wherein resistance coilsin the arms of the Wheatstone bridge comprise heating coils and thecurrent flow established through the bridge by omillations, in theamplifier output connections heat said coils and restore the bridge tothe seaegted temperature, thereby stopping said oscilla- EDGAR PHILIPLAWRENCE WESTELL.

oscillations for restoring the temperature of the

